The present invention relates to a rotary displacement compressor having adjustable regulating means for determining the internal volume ratio of the compressor, and to a method for regulating the internal volume ratio of such a compressor.
The internal compression of a compressor is independent of the pressure in the pressure channel and is, for a certain working fluid, dependent only on the volume ratio of the compressor, i.e. the relation between the volume of a compression chamber at the moment it just has been closed off from the inlet port and the volume of a compression chamber at the moment just before it is opened towards the outlet port. Assuming a constant inlet pressure, a certain volume ratio thus results in a certain pressure in a compression chamber just before it is opened towards the outlet port. This is called the end pressure of the compressor. It is desirable that the internal compression corresponds to the pressure in the pressure channel, so that the pressure in a compression chamber just before it opens towards the outlet port equals the pressure in the pressure channel. If these pressures differ from each other, i.e. at overcompression or undercompression, a rapid flow of gas through the outlet port occurs each time a compression chamber opens towards it, whereby the pressures become equalized. The flow velocity of the working fluid during this short period of time is much higher than the flow velocity of the working fluid when it is displaced out through the outlet port by the rotors, and the direction thereof can be to or from the pressure channel, depending on whether there is overcompression or undercompression. These flow pulses consume power and as a consequence the efficiency of the compressor will decrease. For these reasons, there is always an effort to adapt the built-in volume ratio of the compressor to the pressure in the pressure channel.
In some cases this pressure, however, can vary. This makes it desirable to correspondingly make it possible to vary the volume ratio. It is known to provide a compressor with devices for regulating the volume ratio; so called V.sub.i -regulation. This is accomplished in that the position of the edge of the outlet port, which determines the moment or timing of opening, can be varied in steps or continuously. By this technique, the volume of a compression chamber at the moment of opening can be changed and therewith the volume ratio is changed. In this way it can be achieved that the pressure in said compression chamber roughly equals the pressure in the pressure channel.
Constructively this can be accomplished in many ways, partly depending on which kind of rotary displacement compressor is involved. On, for example, a rotary screw compressor having two cooperating rotors, a frequently used regulating device comprises an axially movable slide, displaceably mounted in guiding means parallel to the rotors. The slide has a surface facing the working space, which surface forms a part of the barrel wall of the working space and complies with its shape. The end of the slide facing the high pressure end of the compressor is provided with an edge forming an edge of the outlet port. When the position of said edge is changed by displacement of the slide, the moment of opening of a compression chamber towards the outlet port will be changed and with this, its volume at that moment is changed.
For adjusting the slide to a correct position, where neither undercompression nor overcompression prevails, it is known to have the slide position influenced by sensed operating parameters of the compressor. Examples of such devices are disclosed in Swedish patent publication Nos. SE 427 063 and SE 430 709, German patent publication No. DD 127 878, U.S. Pat. No. 3,936,239 and International Publication No. WO89/03482. The operating parameters sensed in the compressors disclosed in the above-mentioned patent documents are either the electrical power consumption of the prime mover, the difference between the outlet pressure and the pressure in a compression chamber just before opening, or the flow direction in a channel connecting the working space to the outlet channel. In the first alternative, the slide is adjusted to a position where the power consumption is at its minimum, which corresponds to a minimum of losses in efficiency due to undercompression or overcompression. In the second alternative, the pressure in the compression chamber affects the slide to move in a direction of larger outlet area, whereas the pressure in the pressure channel affects the slide to move in the opposite direction so that these pressures balance each other. In WO 89/03482, a channel connects the outlet channel to a working chamber just before it starts to open towards the outlet port. The flow direction of working fluid in this connection channel is indicative of whether the pressure in the working chamber is higher or lower than the pressure in the outlet channel. The regulating slide is adjusted to a position where the flow through the connection channel is at zero, which means that said pressures are equal.
These known ways of governing the V.sub.i -regulation entail various drawbacks.
Using the power consumption as the governing parameter introduces a source of error in that fluctuations in the electrical supply network affects the sensed parameter. Furthermore, the power consumption as a function of the deviations of the end pressure in the compressor from the pressure in the pressure channel has a very flat characteristic, resulting in a poor accuracy, which allows the influence of said fluctuations to be relatively dominating.
To use the pressure difference for governing the regulation has shown to be difficult to work in practice. The main reason for that is that sensing the end pressure in the compressor cannot be accomplished in a reliable way, since the sensed pressure fluctuates, and considerable pressure pulses are generated each time the means for limiting a compression chamber passes the sensing point. It will therefore be practically impossible to use this technique for reaching a balanced position where neither undercompression nor overcompression prevails.